Capacitive fill level sensors have been used for a long time both as limit switches as well as for media determination. Their basic advantages are that they feature no mechanical moving parts and can measure “through” a housing wall consisting of an electrically insulating material, i.e. sealing problems are avoidable.
Electronic fill level sensors for water-based media have been produced and marketed by the applicant under the designation LMT100, and those for oils and fats under the designation LMT110.
DE102008027921B4 shows an admittance measurement circuit for a capacitive fill level sensor in which a frequency spectrum generated by a tunable signal source is applied to a probe constructed as a resonator and a reference impedance that cannot be influenced by the medium. Its response provides information about the fill level and possible adhesions, but also about the type of medium. Because of the control unit consisting of semiconductor components their permanent operating temperature is limited to 85° C., which in many cases is not sufficient. The problem appears to be solvable by the spatial separation of the probe and the control unit. Since these devices, however, operate at very high frequencies, in a range definitely above 100 MHz, their resonance behavior noticeably worsens, and also their measurement accuracy despite the screened connecting lines.
U.S. Pat. No. 4,259,865A shows such a capacitive fill level measuring device with spatially separated and thus thermally decoupled measuring and reference capacitances. The impedances of both sensors are measured. One of the two sensors is used as a reference for the adjustment of the transmission voltage. An operating frequency is not specified. Because of the voltage drop on the comparatively long lines and their susceptibility to interference, the circuit is inexact and not suited for high frequencies in the range above 10 MHz. The temperature drifts can be compensated by a control voltage VR. Its manufacture is, however, expensive and fault-prone.
U.S. Pat. No. 6,107,924A shows a measuring device for impedance measurement in the frequency range between 10 MHz and 300 MHz. The sensor consists of a transformer-induced resonant circuit adapted to a coaxial cable. The standing wave ratio influenced by the measurement resistor is measured in the coaxial line with a directional coupler.
Here, the quality of the coaxial line, in particular also of the standing waves generated by contact points or plug-in connectors has an influence on the measurement result. The generation of a reference signal is difficult. The measurement principle requires a minimum length of the coaxial line dependent on the frequency. The directional coupler can indeed be in the vicinity of the oscillator but not in the vicinity of the sensor. That initially appears to be an advantage but can require the additional temperature measurement recommended in the literature.